Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Quantas datas de lançamento existem para transmissão digital?
 ###Tabelas: CREATE TABLE current_roster (current_roster_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Number INT, Position VARCHAR, Height VARCHAR, Year VARCHAR, Hometown VARCHAR) CREATE TABLE top_25_lists (top_25_lists_id AUTO_INCREMENT PRIMARY KEY, Year INT, Performance VARCHAR, World Ranking INT, Venue VARCHAR, Date VARCHAR) CREATE TABLE table_of_world_organization_of_the_scout_movement_members (table_of_world_organization_of_the_scout_movement_members_id AUTO_INCREMENT PRIMARY KEY, Country VARCHAR, Membership (from 2010) INT, Name of member organization VARCHAR, Year current Scouting organization joined WOSM VARCHAR, Year member organization was founded VARCHAR, Admits boys/girls VARCHAR) CREATE TABLE league_table (league_table_id AUTO_INCREMENT PRIMARY KEY, Club VARCHAR, Played VARCHAR, Won VARCHAR, Drawn VARCHAR, Lost VARCHAR, Points for VARCHAR, Points against VARCHAR, Points difference VARCHAR, Bonus Points VARCHAR, Points VARCHAR) CREATE TABLE free-to-air_television (free-to-air_television_id AUTO_INCREMENT PRIMARY KEY, Ch № VARCHAR, Channel VARCHAR, Channel content VARCHAR, Transmission VARCHAR, Format VARCHAR, Launch Date VARCHAR, Licence VARCHAR)  
SELECT COUNT Launch Date FROM free-to-air_television WHERE Transmission = Digital